It is not an unusual restriction. I can think of one company that restricts shoppers to no more than 4 of their shops per day (this is up from the 2 per day of a few years back), a number of companies that limit you to 1 per day per client, a company that restricts you to no more than 15 for the client per month and a company that is 1 per client per month self assigned with additional ones possible by special request. For those who feel it is unfair to allow one shopper to do the bulk of shops in a market, these restrictions must seem like a godsend.
It is probably a good practice for an MSP to do this as it means they have a variety of shoppers working and will always have the opportunity for new shoppers to have a chance to work. Considering the short 'lifetime' of most shoppers, it really is important for companies to have new shoppers coming along so when their longer term shoppers decide to throw in the towel they aren't having all newbies suddenly.